Solid polymer electrolyte fuel cells (SPFCs) have been shown to be reliable for generating electricity by the oxidation of a conventional fuel such as hydrogen. The long demonstrated life and relative simplicity of design make SPFCs particularly suitable for space and transportation applications.
A single solid polymer electrolyte fuel cell comprises an ion exchange membrane separating an anode and a cathode, all of which is interposed between electrically conductive separator plates. A plurality of cells make up an SPFC stack.
The anode and cathode in a solid polymer electrolyte fuel cell are planar in configuration, and are normally formed of porous electrically conductive sheet material such as carbon fiber paper. A suitable catalytic material, such as finely divided platinum, is typically applied to the surfaces of the anode and cathode facing the membrane to render the portions containing the catalytic material electrochemically active.
Typically, flow field grooves are molded or machined on the surfaces of the electrically conductive separator plates facing the anode or the cathode to accommodate reactant fluid distribution and reaction product collection and elimination.
In conventional SPFCs, the solid polymer membrane serves at least three functions. First, the membrane separates the anode from the cathode. Hydrogen fuel is oxidized at the anode to form protons (hydrogen cations), which migrate across the membrane to the cathode. Oxygen is reduced at the cathode and reacts with the migrated hydrogen cations to form water. Second, the electrochemically active region of the membrane serves as a medium through which the hydrogen cations migrate to the cathode. Third, the portion of the membrane extending beyond the electrochemically active region into the space between the separator plates serves as a gasket to prevent reactant gases from escaping to the atmosphere from between the separator plates.
An advantage of solid polymer membranes is their immiscible nature, which facilitates the separation and removal of reaction products. Other advantages of solid polymer membranes include their relative insensitivity to differential pressure between the anode and the cathode, their chemical stability and their non-corrosiveness.
A disadvantage of solid polymer membranes is their high cost. This cost is even greater in SPFCs where the membrane itself is used as a gasket, because more membrane area is required. Where the membrane serves as a gasket, the membrane must extend substantially beyond the electrochemically active region of the membrane and into the space between the graphite separator plates. That portion extending beyond the active region adds to the overall cost of the SPFC, but is not utilized as a medium for cation migration.
Examples of SPFCs in which the solid polymer membrane serves as a gasket include those developed and described by United Technologies Corporation (UTC) for zero gravity applications, rigorous naval applications, and extraterrestrial surface applications. In such UTC fuel cells, a portion of the solid polymer membrane is interposed between the anode frame and the cathode frame and functions as a gasket, preventing reactant gases from escaping to the atmosphere.
There are several disadvantages to configurations employing the membrane itself as a gasket. As already noted, the cost of solid polymer membranes is high, and using a portion of the membrane as a gasket requires a larger membrane area, thus increasing the overall cost of the fuel cell. Use of the membrane as a gasket also exposes the membrane edge to the atmosphere, thereby allowing the evaporation of water, required for effective cation transport, from the membrane. In addition, the gasketing portion of the membrane is in contact with the separator plates at about 70.degree. C.-80.degree. C. (158.degree. F.-176.degree. F.), thus further promoting the dehydration of the membrane edge and possible degradation of the membrane's physical and chemical properties. For example, contaminants such as various metal ions can leach out from the separator plates and diffuse through the portion of the membrane acting as a gasket to the electrochemically active portion of the membrane, thus reducing the membrane's ability to act as an ion exchange medium. Another disadvantage of SPFCs in which the membrane serves as a gasket is that, where the membrane, in its protonated form, contacts the separator plates, the acidic membrane will corrode the separator plates.